This invention relates to a radio receiver, and in particular to a radio receiver for use in a digital Time Division Multiple Access (TDMA) communication system, such as the GSM cellular radio telephone system.
In a transmitter of a digital radio communications system, information bits are mapped to waveforms that modulate a carrier signal. In the receiver, the transmitted sequence of bits is retrieved by demodulation of the received signal.
A generally efficient design of receiver circuit is the direct conversion, or homodyne, radio receiver, in which the received carrier signal is directly downconverted to baseband, without use of any intermediate frequencies. This architecture can be efficient in terms of cost, size and current consumption.
One problem which can arise in a direct conversion receiver is distortion due to a DC offset. DC offset can arise in the baseband or radio parts of the transmitter, or, more commonly, in the baseband or radio parts of the receiver circuit. More specifically, the main causes of DC offset in the receiver are: transistor mismatch in the signal path; the local oscillator signal leaking into the received signal and self-downconverting to DC in the mixer; and a large near-channel interferer leaking into the local oscillator and self-downconverting to DC.
As a result of these sources, the DC offset signal can in fact be several dB larger than the magnitude of the information signal. It is thus apparent that the DC offset must be removed before the data can be satisfactorily recovered.
An important part of the demodulation process in a digital TDMA receiver is the synchronization. The purpose of synchronization is to find the position of the symbol in a received signal burst. This is made possible by transmitting a specific symbol pattern known as the training sequence within the burst. The training sequence is known to the receiver. Then, the receiver can carry out a correlation between the received signal and the known training sequence, in order to find the synchronization position. The training sequences used within the system are designed in such a way that the synchronization performance is optimised when the received burst is a filtered version of the transmitted symbols plus white noise.
Thus, a received radio signal is downconverted, low-pass filtered, and sampled in a radio receiver front-end circuit. As is known, the sampled signal y(t) can be described by:                               y          ⁢                      xe2x80x83                    ⁢                      (            t            )                          =                  xe2x80x83                ⁢                              h            ⁢                          xe2x80x83                        ⁢                          (              0              )                        ⁢                          xe2x80x83                        ⁢            u            ⁢                          xe2x80x83                        ⁢                          (              t              )                                +                      h            ⁢                          xe2x80x83                        ⁢                          (              1              )                        ⁢                          xe2x80x83                        ⁢            u            ⁢                          xe2x80x83                        ⁢                          (                              t                -                1                            )                                +          …          +                      h            ⁢                          xe2x80x83                        ⁢                          (              L              )                        ⁢                          xe2x80x83                        ⁢            u            ⁢                          xe2x80x83                        ⁢                          (                              t                -                L                            )                                +                      e            ⁢                          xe2x80x83                        ⁢                          (              t              )                                                              =                  xe2x80x83                ⁢                              HU            ⁢                          xe2x80x83                        ⁢                          (              t              )                                +                      e            ⁢                          xe2x80x83                        ⁢                          (              t              )                                          
where H=[h(0), . . . , h(L)] is a complex valued vector representing the radio channel, and U(t)=[u(t), . . . , u(t-L)] is a complex valued representation of the transmitted symbol at time t, txe2x88x921, . . . , t-L. The signal e(t) is assumed to be white noise.
The sampled signal y(t) is then supplied to a synchronization unit, that correlates the received burst with a training sequence, in order to find the synchronization position. Based on the found synchronization position, a channel estimation unit then estimates the coefficients H which define the radio channel. The estimated channel is then supplied to an equaliser that decodes the received data.
However, in the presence of DC components in the signal which is to be demodulated, the standard synchronization procedure is no longer optimal, and thus the bit error rate in signals received by the receiver may be worsened.
The present invention relates to a synchronization procedure that provides acceptable performance when the received signal includes a DC offset.
More specifically, according to the invention, the DC offset component is estimated in two stages.
In preferred embodiments of the invention, a coarse DC estimation is performed, and the estimate is removed from the received sequence. Then, a coarse synchronization is performed in order to estimate the synchronization position within the burst. Subsequently, a coarse channel estimation and a finer DC estimation are performed simultaneously, and the refined DC estimate is removed. Finally, a refined synchronization and channel estimation are performed, and the refined estimate of the channel is fed to an equaliser, which acts on the received signal, after removal of the refined estimate of the DC component.